Which functional group participates in disulphide bond formation in proteins?

$1.78 \ g$ of an optically active $L$-amino acid $(A)$ is treated with $NaNO_{2} / HCl$ at $0^{\circ}C$. $448 \ cm^{3}$ of nitrogen at $STP$ is evolved. $A$ sample of protein has $0.25 \%$ of this amino acid by mass. The molar mass of the protein is

The molecular weight of protein is

The correct match between Item $I$ and Item $II$ is

Item $I$	Item $II$
$A$. Ester test	$P$. Tyr
$B$. Carbylamine test	$Q$. Asp
$C$. Phthalein dye test	$R$. Ser
	$S$. Lys

Which of the following is not an essential amino acid?

$A$ tetrapeptide is made of naturally occurring alanine,serine,glycine,and valine. If the $C$-terminal amino acid is alanine and the $N$-terminal amino acid is chiral,the number of possible sequences of the tetrapeptide is $....$